2011; Lamichhaney et al. 2012; Limborg et al. 2012; DeFaveri et al. 2013); ocean connectivity has been correlated with genetic divergence in herring (Teacher

et al. 2013) as has temperature for herring and three-spined stickleback (Limborg et al. 2012; DeFaveri et al. 2013). Additional factors that have been demonstrated to affect genetic structure include larval development and dispersal (Kyle and Boulding 2000). For example, the free-floating larval stage in Atlantic herring and a later pelagic life stage mediate potential for long distance dispersal and is a likely explanation for the lack of genetic structuring for herring within the Baltic Sea shown here, as well as in previous studies using neutral genetic markers (Bekkevold et al. 2005; Jørgensen et al. 2005). Genetic divergence among herring populations has indeed been shown to be affected more by ocean TGF-beta inhibitor currents than geographic

distance (Teacher et al. 2013). Ocean currents are more likely to affect species with freefloating life stages, such as herring, or bladderwrack, for which dispersal of eggs are limited, but detached adults have the potential for dispersal by means of rafting (Tatarenkov et al. 2007). Species with stationary development on the other hand, such as European whitefish and Northern pike, which are both associated with freshwater spawning, are likely to have more limited dispersal. The observed pattern of ACP-196 cost isolation by distance found in whitefish and pike in the present study as well as previous studies (Laikre et al. 2005b; Olsson et al. 2012a) is consistent with such limited dispersal and suggests that migration predominantly takes place between geographically proximate populations. It should be noted that recent studies have detected isolation by distance also in herring (Teacher et al. 2013) and three-spined and nine-spined stickleback (DeFaveri et al. 2012). Those studies included

not larger sample sizes and/or more genetic markers than examined here, however, and may thus have been characterized by higher statistical power for detection of isolation by distance. Other factors potentially affecting genetic diversity in the Baltic Sea include postglacial colonization of the area by different phylogenetic lineages. Nine-spined stickleback in the Baltic Sea has been shown to consist of one western and one eastern lineage meeting roughly at the entrance of the Baltic Sea (Shikano et al. 2010; Teacher et al. 2011), as previously also shown for cod (Nielsen et al. 2003) and the bivalve Macoma balthica (Luttikhuizen et al. 2012). A more extreme example of transition zones is represented by the blue mussel, where the species M. trossulus, native to the Baltic Sea is hybridized with M. edulis (Riginos and Cunningham 2005).